Project Summary/Abstract Pregnancy impacts the maternal immune system and breast cancer risk, but whether these effects are related is unknown. Many women are immunologically primed to fetal antigens during pregnancy due to the unique expression of paternally- inherited antigens by the fetus. However, whether this priming has any consequence on subsequent progression of diseases in women is unknown. Tumor antigens are proteins that have limited expression in normal adult tissues, but are strongly expressed in many cancers, including cancer of the breast. Because of their ability to induce immunity, there is strong scientific and medical interest in exploiting this property for development of anti-cancer vaccines. Despite their restricted expression patterns, several, which we call shared placenta/tumor antigens, are highly expressed in the placenta. Investigators have long speculated that trophoblast cells and tumor cells share similarities, but the immunologic relationship between shared placenta/tumor antigens, breast cancer, and pregnancy are completely unknown. In this proposal, we explore the hypothesis that shared tumor/placenta antigens are immunogenic during pregnancy. These experiments serve as critical proof of concept experiments, with which we can further explore the long term hypothesis that immune priming to shared placenta tumor antigens during pregnancy alter breast cancer risk in parous women. To determine whether the placenta primes the maternal immune system against shared placenta/tumor antigens, we propose the following Specific Aims: 1. Determine whether the murine maternal immune system is primed to shared tumor/placenta antigens. 2. Test women for evidence of sensitization to shared placenta/tumor antigens. To our knowledge, this is the first time that a long-term consequence of the physiologically normal maternal immune response to fetal antigens will be considered in light of a critical clinical problem: women's cancer. In the long-term, this work could profoundly impact our understanding of why parity changes risk of breast cancer, and lead to insights into novel cancer prevention and therapeutic strategies.